5.7 swap questions?

Not to derail the conversation too much, but does anyone know the difference between the older 5.7's and the newer ones? The main thing I'm talking about is the HP difference. The original 5.7's were 345 or something like that, but the newer ones they're putting out in the past year or so are rated at around 380. Were there any changes made to the engine or was it just a factory tuning thing? I'm looking at the 5.7 swap myself and plan to put a cam and maybe 6.1 heads on one but don't know if I should be looking for a specific year range of engine or not.